The psalmist is beckoning the stars to sing. The idea is for us to appreciate the physical world God has created for us. It is like saying, "Look up in the sky and see how beautiful it is. Let us thank God for creating such a beautiful sky for us to appreciate."
Notice the phrase "waters above the skies" in verse 4. In Genesis 1:7, there is a reference to waters being above the sky that came down when the flood happened. Scholars believe there was a water vapor that was part of the earth's atmosphere at one time.
Apparently, the psalmist understood that waters did and maybe still exist outside of the world that we can touch or see. The psalmist asks us to be grateful for the way God did create the world, for our benefit, both how it was, and how it is.
Finally, verse 6 says that, "they will never pass away". It is important to state that this does not refer to God's creation, but to His word. If our world had a beginning, then it will also have an ending one day. However, the word of God lives longer than that long time span. We should appreciate His word, if for no other reason, because it lasts forever.